How to enter grace notes

The best way to enter grace notes is to select Grace from the voice palette
at the far right of the Score Editor and then enter the grace notes where
you like. This way the timing of the grace notes will not interfere with
the timing of the rest of the notes in the measure. Grace notes entered
this way do not generate any rests.

If you want to enter grace notes before the first note of the measure,
you can enter the grace notes at the beginning of the measure, and then
with the spacing tool move the non-grace notes on the first beat over
a bit so they are not obscured by the grace note.